When fuel is added to a fuel reservoir, such as the gasoline tank of an automobile from a conventional gas dispenser apparatus such as the dispensing nozzle of a gasoline dispenser, gasoline vapor is displaced from the gasoline tank. If the vapor is not collected in some way, it will be released into the atmosphere. Due to the large number of automobile refuelings, such releases of fuel vapor constitute a significant hazard to the environment, particularly in heavily populated areas. Releases of these vapors which are composed of volatile organic compounds (VOC's) such as hydrocarbons, are presently the subject of significant and increasing federal and local regulation.
Generally, such evaporative emissions result from the venting of fuel vapors from the fuel tank due to diurnal changes in ambient pressure and/or temperature, the vaporization of fuel by a hot engine and/or exhaust system, and the escape of fuel vapors during refueling of the vehicle. The venting of fuel vapor from the fuel tank due to diurnal pressure and/or temperature changes (i.e., diurnal emissions) is responsible for a majority of evaporative emissions. Diurnal changes in pressure and/or temperature cause air to flow into and out of the fuel tank. Air flowing out of the fuel tank inevitably carries fuel vapor, which is created by the evaporation of fuel into the air contained above the fuel within the fuel tank. If this flow of air is left untreated and is allowed to escape directly into the atmosphere, undesirable emissions occur.
Motor vehicle manufacturers have reduced the level of diurnal emissions through the use of evaporative canisters. Generally, an evaporative canister has a vapor inlet, a purge port, and a vent port. The vapor inlet is fluidly connected by a vapor conduit to the air space in the fuel tank. Diurnal changes in pressure and/or temperature cause air within the fuel tank to flow through the vapor conduit and into the evaporative canister via the vapor inlet. The air carries fuel vapor and/or hydrocarbons. The evaporative canister contains a sorbent material, such as an activated carbon, that strips fuel vapor from the air as it flows through the canister. The treated air then flows out the vent port and into the atmosphere. The purge port is fluidly connected by a valved purge conduit to the combustion air intake of the motor vehicle engine. When the engine is running, the combustion air intake is at sub-atmospheric pressure, and the valve is opened to thereby connect the purge port to the combustion air intake. Fresh air is drawn by the sub-atmospheric pressure through the vent port and into the evaporative canister. The fresh air flows through the sorbent material, out the purge port and into the combustion air inlet. The flow of fresh air through the evaporative canister strips sorbent material of stored fuel vapor and/or hydrocarbons, thereby purging the evaporative canister of hydrocarbons.
DE 36 09 976 C2 discloses an activated carbon filter for a fuel supply system of a motor vehicle which includes a container filled with activated carbon particles. This container includes at one side an opening for the admission of fresh air which opening is covered by filter material. At the opposite side of the container, means are provided for the connection of the activated carbon filter to the fuel supply system. In the area of the opening for the admission of fresh air to the activated carbon filter, an electric heating arrangement is provided in the form of a metallic grid structure. When the engine is shut down, hydrocarbon-containing gas evaporates from the fuel supply system and reaches the activated carbon filter, which adsorbs the hydrocarbons. As the engine is started, fresh air flows into the activated carbon filter through the respective opening. The fresh air is preheated by the heating arrangement, whereby the activated carbon filter is regenerated as soon as engine operation begins and the hydrocarbons adsorbed by the activated carbon particles are released and supplied to the internal combustion engine for combustion therein.
It is further known from DE 195 14 887 A1 to provide in air cleaning systems a thin filter layer consisting of an adsorbing cover fleece which may contain activated carbon.
Other systems and methods for trapping volatile hydrocarbon fuel vapors, from the fuel tank of an automobile are also well known. One such system is typically referred to as an evaporative loss control system and relys on a canister containing a regenerable adsorbent such as activated charcoal. The adsorbent adsorbs the volatile hydrocarbons and when engine operating conditions are appropriate for combusting the trapped hydrocarbons, a stream of air is passed through the adsorbent to desorb the adsorbent and the hydrocarbon-laden air stream is passed into the engine where the desorbed hydrocarbons are combusted. Exemplary U.S. patents disclosing evaporative loss control systems include the following: U.S. Pat. Nos. 4,877,001; 4,750,465; and 4,308,841.
However, the storage capacity of the active charcoal filter drops continuously with an increase in the quantity of the stored hydrocarbons and it is therefore necessary to regenerate the active charcoal filter at regular intervals; that is, it is necessary to again remove the stored hydrocarbons from the active charcoal filter. For this purpose, the active charcoal filter is connected via a regeneration valve to an intake manifold of the engine which functions to induct combustion air. By opening the regeneration valve, a pressure drop develops between the active charcoal filter and the intake manifold by means of which the hydrocarbons, which are stored in the active charcoal filter, are conducted into the intake manifold in order to finally be combusted in the engine and thereby be disposed of.
Due to incomplete desorption of the hydrocarbons, minute levels of hydrocarbons may remain stored in the adsorbent material of a purged evaporative canister. The term “heel” as used herein refers to residual hydrocarbons generally present on an adsorbent material when the canister is in a purged or “clean” state and may result in a reduction of the adsorption capacity of the adsorbent. Bleed emissions, on the other hand, refer to emissions that escape from the adsorbent material. Bleed can occur, for example, when the equilibrium between adsorption and desorption favors desorption significantly over adsorption. The heating of the fuel tank may causes air to flow from the fuel tank, through the canister, out the vent port and into the atmosphere. If conditions favor desorption (e.g., incomplete desorption of the adsorbent, thereby lowering adsorption capacity), the air may carry bleed emissions out of the canister and into the atmosphere.
For economic reasons, use of activated carbon dominates commercially, either in the form of a canister filled with extruded granules or a monolith extruded with carbon. However, as worldwide evaporative emission regulations become more and more stringent, the technical limitations of carbon-based systems become more and more apparent. In particular, an excessively high “bleed” rate or the formation of a permanent hydrocarbon heel often make the design and use of carbon-based systems problematic. As a result, the development of new materials and systems is desired.
Zeolite-based systems are a possible alternative to those of carbon for controlling evaporative emissions from the fuel storage system of motor vehicles. Although zeolites are generally more expensive than carbon, the hydrocarbon adsorption properties of zeolites may overcome the inherent bleed rate and heel formation problems associated with carbon. The use of zeolites materials for adsorbing uncombusted hydrocarbons in the exhaust gas stream of an automobile is also well known. These systems and methods are particularly useful for adsorbing uncombusted hydrocarbons emitted during the cold start of the automobile engine.
For example, U.S. Pat. No. 4,985,210 is directed to an exhaust gas purifying apparatus for an automobile employing a three-way catalyst with either a Y-type zeolite or a mordenite used in a hydrocarbon trap upstream of the three-way catalyst. In the embodiment of FIG. 2 of U.S. Pat. No. 4,985,210, a bed of activated carbon is disposed upstream of an adsorbent zone. A solenoid-operated valve mechanism serves to direct the exhaust gas stream either through or around the activated carbon bed, depending on the temperature of the exhaust gas stream, and then through the adsorbent zone and the three-way catalyst.
U.S. Pat. No. 5,051,244 is directed to a process for treating an engine exhaust gas stream in which the gas stream is directed through a molecular sieve in an adsorbent zone during the cold-start phase of engine operation. When the hydrocarbons begin to desorb, the adsorbent zone is by-passed until the catalyst is at its operating temperature, at which point the gas stream is again flowed through the adsorbent zone to desorb hydrocarbons and carry them to the catalyst zone. A paper by M. Heimrich, L. Smith and J. Kotowski entitled Cold-Start Hydrocarbon Collection for Advanced Exhaust Emission Control, SAE Publication Number 920847, discloses an apparatus which functions in a manner similar to that of U.S. Pat. No. 5,051,244.
U.S. Pat. No. 5,125,231 discloses an engine exhaust system for reducing hydrocarbon emissions, including the use of beta zeolites as hydrocarbon adsorbents. Zeolites having a silica/alumina ratio in the range of 70/1 to 200/1 are preferred adsorbents. The apparatus includes by-pass lines and valves to direct exhaust gases from a first converter directly to a second converter during cold-start operation and when the first converter reaches its light-off temperature, to either by-pass the second converter or recycle effluent from it to the first converter.
U.S. Pat. No. 5,158,753 discloses an exhaust gas purifying device comprising: a catalyst device installed in the exhaust gas path of an internal combustion engine for treating the exhaust gas of the engine; an adsorbing device installed in the exhaust gas path between the catalyst device and the internal combustion engine, for treating the exhaust gas of the engine. One embodiment includes a heat exchanger for performing heat transfer between the exhaust gas flowing from the internal combustion engine to the adsorbing device and the exhaust gas flowing from the adsorbing device to the catalyst device. Alternatively, the catalyst device includes a catalyst secured in the low-temperature-side gas flow path of a heat exchanger, and the exhaust gas flowing from the internal combustion engine to the adsorbing device is allowed to flow to the high-temperature-side gas flow path of the heat exchanger.
U.S. Pat. No. 6,171,556 discloses a method and apparatus for treating an exhaust gas stream containing hydrocarbons and other pollutants. The method comprises the steps of flowing the exhaust gas stream through a catalytic member comprising a monolith body having a first catalyst zone and a second catalyst zone therein to contact a catalyst in a first catalyst zone to convert at least some of the pollutants in the exhaust gas stream into innocuous products. The exhaust gas stream is then discharged from the catalytic member and flowed through an adsorbent zone to adsorb at least some of the hydrocarbon pollutants with an adsorbent composition. The exhaust gas stream is discharged from the adsorbent zone and flowed to the second catalyst zone to convert at least some of the pollutants into innocuous products. The exhaust gas stream, so treated, is then discharged to the atmosphere through suitable discharge means. A preferred adsorbent is a zeolite, having a relatively high silica to alumina ratio and a low relative Bronsted acidity. The preferred adsorbent compositions comprise beta zeolites.
As discussed above, zeolites are often used as coatings on monolithic substrates for various high temperature adsorption and catalytic applications. In these cases, inorganic binder systems are used that survive exposure to high temperatures (e.g., >500° C.) and provide good coating adhesion. However, for low temperature application (e.g., <500° C.), inorganic type binders are often not suitable since their binding characteristics are severely diminished. In these low temperature applications, organic polymer binders are ideal since they are structurally stable and provide excellent coating adhesion. This is accomplished by the addition of suitable stabilizing agents to the slurry formulation.
For example, commonly assigned U.S. Patent Publication No. 2004/0226440, incorporated herein by reference, discloses a hydrocarbon adsorption unit. The unit is positioned in the air intake system of an automobile engine and has an air intake and air outlet. According to the application the adsorber material may be silica gel, a molecular sieve and/or activated carbon and contains an organic polymer binder, as well as an anionic, nonionic or cationic dispersant, that will cause the material, preferably as an aqueous slurry, to adhere to the surface of a substrate.
However, without proper choice of these stabilizing agents, interparticle agglomeration of zeolite particles or coagulation of zeolite and binder particles will occur, thus rendering the slurry unstable for coating application. As a result, a zeolite-based coating formulation must be developed that not only has good adhesion (particularly to metal substrates) at low temperature, but also excellent adsorption characteristics.
Therefore, it is an objective of the present invention to provide an improved evaporative emission control system for controlling evaporative emissions from a motor vehicle's fuel storage system.